Extensive research and development has been carried out for using metallic lithium, which can achieve high energy density at high voltages, as a negative electrode for a non-aqueous electrolyte secondary battery. However, using metallic lithium as the negative electrode causes precipitation of dendritic lithium (dendrite) on the surface of the negative electrode during charging, so that the charge/discharge efficiency of the battery decreases. Furthermore, the dendrite may pierce through the separator and comes in contact with the positive electrode, thus causing the problem of internal short circuit. Therefore, lithium ion secondary batteries using, for their negative electrodes, carbon materials (for example, graphite) capable of reversibly absorbing and desorbing lithium have been put into practical use. Although carbon materials have a smaller capacity than metallic lithium, they are superior in terms of the safety and the cycle life.
However, the capacity of negative electrodes in practical use is about 350 mAh/g. This capacity is already close to the theoretical capacity (372 mAh/g) of graphite. Therefore, there is a limit to a further increase of the capacity of negative electrodes using graphite. On the other hand, in order to secure energy sources for future high-function portable devices, there is a demand for negative electrodes for which a further capacity increase has been achieved. For this purpose, a negative electrode material having a higher capacity than graphite is required.
Therefore, negative electrodes using an alloy are currently gaining attention. For example, an alloy including silicon or tin causes an electrochemical reversible reaction with lithium ion. Furthermore, some metallic elements have a very larger theoretical capacity than graphite. For example, the theoretical discharge capacity of silicon is 4199 mAh/g, which is 11 times that of graphite.
However, silicon and tin form a lithium-silicon alloy and a lithium-tin alloy when they react with lithium. At that time, their crystal structures undergo a change, so that the negative electrode experiences a very large expansion. For example, when silicon absorbs lithium to a maximum extent, it theoretically expands 4.1 times its initial volume. Graphite, for which intercalation reactions are utilized, expands only about 1.1 times, since lithium is intercalated between graphite layers.
Due to such expansion, a large stress is induced in the negative electrode. For this reason, the active material cannot be sufficiently fixed to a current collector with a binder, which is typified by polyvinylidene fluoride (PVDF) and styrene-butadiene rubber (SBR). Accordingly, there may be cases where the active material is detached from the current collector, or the contact points between the active materials are reduced. As a result, the internal resistance in the negative electrode increases to cause a reduction in the current collection properties, which also results in a reduction in the cycle characteristics. To prevent this, it is conceivable to increase the amount of the binder. However, if the amount of a material that does not contribute to charge/discharge increases, then the discharge capacity of the negative electrode decreases. Moreover, when a large amount of a non-conductive material is mixed in the negative electrode, the internal resistance in the negative electrode increases. Accordingly, the high rate discharge characteristics and the cycle characteristics decrease eventually.
Therefore, it has been proposed to form a film of a negative electrode active material comprising amorphous silicon on a current collector whose surface has been roughened (Patent Document 1). This proposal is intended to achieve a firm bonding between the active material and the current collector, thus preventing a reduction in the current collection properties and the cycle characteristics. However, according to the proposal of Patent Document 1, expansion of silicon during absorbing lithium can be allowed only in the thickness direction. Therefore, during charging (expansion), the active material particles press against each other, causing the electrolyte to be squeezed out from the active material layer. As a result, only the outermost surface of the negative electrode can come into contact with the electrolyte at the final stage of charge and the initial stage of discharge, so that electrochemical reaction is inhibited.
Furthermore, it has been proposed to place a mesh on a current collector before depositing the active material at the time of forming the negative electrode (Patent Document 2). This proposal is intended to dispose plural island-shaped deposited films that are separated from each other. With such a negative electrode, the electrolyte is retained without being squeezed out from the active material layer at the time of expansion. However, due to a large thickness of the mesh, the distance between the island-shaped deposited films is very large, so that wasted space is created inside the negative electrode. Moreover, the active material comes under the mesh, and it is therefore difficult to form plural deposited films, while separating films from each other and reducing the distance between the films. Consequently, the negative electrode has a very low capacity, which counteracts the advantage of the high capacity of the active material (for example, silicon).
Furthermore, it has been proposed to form an active material layer on a current collector, and then to form voids in the thickness direction in the active material layer by etching (Patent Document 3). This proposal is intended to divide the active material layer into plural minute regions. However, the effect of etching is greatly influenced by the surface roughness of the current collector, and is very difficult to control. Furthermore, in the case of chemical etching, many oxides are formed on the surface of the active material layer, so that there is concern that the reaction between the electrolyte and the active material may be inhibited. Moreover, in the case of using chemical etching, a phenomenon (undercut phenomenon) occurs in which a portion of the active material layer that is under the mask is also etched. Consequently, each of the minute regions has the shape of an inverted cone in which its portion in the vicinity of the current collector is cut away in a greater amount, so that breaking tends to occur in the vicinity of the current collector at the time of expansion.
Patent Document 1 Japanese Laid-Open Patent Publication No. 2002-83594
Patent Document 2 Japanese Laid-Open Patent Publication No. 2002-279974
Patent Document 3 Japanese Laid-Open Patent Publication No. 2003-17040